Picocyanobacteria
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Picocyanobacteria are the smallest of cyanobacteria cells, often around one thousandth of a millimeter in
diameter; however, they typically occur in high concentrations (the term pico refers to cells smaller than
2 mm). Two of the genera Synechococcus and Prochlorococcus are sowidely distributed in the open ocean that
they are probably the most abundant photosynthetic cell types in the biosphere. Freshwater picocyanobacteria
are usually ascribed to the genus Synechococcus, although this is an artificial taxonomic category that
encompasses diverse genotypes and phenotypes. They are readily observed by epifluorescence microscopy,and increasingly by flow cytometry, and comparative studies have shown that they contribute a large proportion
of total photosynthetic biomass in oligotrophic lakes. Their high surface-to-volume ratio confers an
advantage in low nutrient conditions, and their small size can also be an advantage for light capture because
of a lack of internal shading (‘package effect’). These minute cells provide a nutrient and energy source
to flagellates and ciliates in the microbial food web, and they are also subject to loss by viral attack
(cyanophage).